RESAGK, Christian 1 ; DIETHOLD, Christian 2 ; FRÖHLICH, WERNER, Michael 3 ;

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1 Lorentz Force Velocimetry for Poorly Conducting Fluids - Development and Validation of a Novel Flow Rate Measurement Device WEGFRASS, Andre 1 ; RESAGK, Christian 1 ; DIETHOLD, Christian 2 ; FRÖHLICH, Thomas 2 ; WERNER, Michael 3 ; HALBEDEL, Bernd 3 ; THESS, André 1 ( 1 Institute of Thermodynamics and Fluid Mechanics 2 Institute of Process Measurement and Sensor Technology 3 Institute of Inorganic-nonmetallic Materials Department of Mechanical Engineering, Ilmenau University of Technology, P.O. Box , Ilmenau, Germany) Abstract: The paper demonstrated the feasibility of Lorentz Force Velocimetry for flow rate measurements of weakly conducting electrolytes using experimental results on salt water flow exposed to a permanent magnet system. This innovative flow measurement technique allows the non-contact determination of flow rates and relies on the interaction between a magnetic field and a moving conducting fluid. When an electrically conducting fluid moves through the magnetic field a Lorentz force is generated and acts on the measurement system. The present report provides an overview about the experimental setups and the first measurement results. Key words: Lorentz force velocimetry; flow measurement; electrolytes; non-contact measurement 1 Introduction and Motivation There are several well-known techniques available to accomplish the various required flow measurement tasks in industry. But in some special applications the fluids are extremely hot, corrosive or abrasive and classical methods would not withstand the rough environment for a sufficiently long time, because the sensor in each of these techniques interacts with the surrounding fluid or with the pipe wall. Examples for these fluids include metal melts and acids. In order to avoid these interactions a novel contact-free flow rate measurement device has been developed, the so-called Lorentz force flowmeter (LFF). The basic idea of the measurement device relies on the interaction between a magnetic field and a moving conducting fluid. When an electrically conducting fluid moves through the magnetic field, a Lorentz force is generated that acts on the measurement system. The higher the electrical conductivity of the fluid, the higher the generated Lorentz force, and in turn the higher the force acting on the measurement device. So far, this method has been studied [1] [2] and verified [3] only for molten metals, as they have a conveniently high electrical conductivity of several MSm -1, and is known as Lorentz force velocimetry (LFV). The work of our group extends the known LFF for molten metals to an LFF that is capable of resolving the tiny Lorentz forces involved in the flowmetering of poorly conducting electrolytes. In the course of the project work, two different test facilities have been built. Both facilities use salt water as the

2 weakly conducting model fluid. 2 Experiment and Results The two facilities mentioned in the previous section can be distinguished by their means of force measurement; the preliminary setup uses the displacement of a pendulum (1) for detecting the Lorentz force, the final setup uses electromagnetic force compensation (2). Both principles will be explained in the following. (1) Pendulum Setup (2) (1) (3) flow Fig. 1: Sketch of the pendulum setup. The main components are the pendulum comprising the magnet system (1), the interferometric deflection measurement device (2), and the electrolyte channel of rectangular cross-section (3). To prove the applicability of LFV to weakly conducting fluids, we perform experiments on the setup presented in Fig. 1 that contains salt water with an electrical conductivity in the range of 2.3 Sm -1 to 14 Sm -1 at room temperature. Here, the measurement system based on the principle of deflection. The fluid flows in a duct with rectangular cross-section (30 mm x 50 mm), whose design is based on the numerical results of Werner et al. [4].The flow velocities are in the range of up to 4 ms -1. In order to generate a high magnetic field strength, we apply a magnet system which is equipped with high-energy magnets made of NdFeB. The magnet system is suspended on thin tungsten wires resembling a pendulum and the entire system is mounted on a frame of aluminium profiles. The generated Lorentz force causes a deflection of the magnet system which is detected with a laser interferometer. Both the aluminium frame and the laser interferometer are mounted on a granite block with a mass of about 400 kg in order to suppress vibrations from the environment. The Lorentz force is calculated from the dimensions of the pendulum and the measured average deflection [5] (Fig. 2). The mentioned elements above are crucial for measuring the Lorentz force which is expected to be in the range of FLorentz 10-5 N [4] from numerical simulations. In order to evaluate the sensitivity of LFF, we perform experiments at various fluid velocities and for five different electrical conductivities of 2.3 Sm -1, 6.5 Sm -1, 8.5 Sm -1, 13 Sm -1 and 14 Sm -1. The experimental results are compared with numerical simulations (see Fig. 3), produced by three-dimensional simulation software MAXWELL. Beside the fact, that the results are in good

3 agreement with each other, it can also be seen that there is a linear dependence of the Lorentz force on the flow velocity in both experiment and numerical simulation. This is consistent with the theory of Thess et al. [1]. pump on pump off BP1 BP2 BP3 Fig. 2: Diagram depicts an example of the step response behavior of the LFF during a turn on/off scenario of the salt water flow. Here, the salt water has an electrical conductivity of 6.5 Sm -1 and underlies an acceleration between 0 ms -1 (pump off) and 3.0 ms -1 (pump on). Furthermore, the filtered red curve indicates an increasing basic-plateau (BP - no fluid flow) or respectively a drift, causing by the heat transmission from pump. The encouraging result of this comparison is that it proves the applicability of LFV to weakly conducting electrolytes. However, it must be emphasized that this preliminary experimental setup is not an optimized fluid flow design and does not provide any information on the flow dynamics inside the duct and their influence on the total Lorentz force. In order to overcome these disadvantages, a new measurement system has been built F in µn u in m/s Fig. 3: Measurement results of the pendulum experiment. The graph shows the relationship between the velocity of the salt water in the test section and the measured Lorentz force which is acting on the magnet system. The different line types and

4 symbols represent different electrical conductivities from 2.3 Sm-1 (rectangle), 6.5 Sm-1 (triangle-down), 8.5 Sm-1 (triangle-up), 13 Sm-1 (circle) and 14 Sm-1 (cross) with the corresponding numerical simulation results (lines). (2) EMF Setup (3) (2) (1) (4) Fig. 4: Left: Sketch of the EMF-setup. The main parts are the LFF / EMF balance (1), the well-defined fluid channel made out glass fiber reinforced plastic (2) with the 1.5 m long test section (3) and a magneto-inductive flowmeter (4) working as reference measurement device. Right: Picture of the experimental setup. In order to overcome the disadvantages from the pendulum setup an improved setup has been built. Besides the new measurement method which is applied to the new setup (so-called EMF balance; acronym for electromagnetic force compensated balance), the new fluid channel enables the possibility to adjust the fluid profile within the measurement test section (turbulent mean profile or laminar profile, obstacles, etc.). That means the new setup helps to understand the effect of the flow profile on the total Lorentz force. This is important because extensive numerical simulations show an influence of the total Lorentz force in the order of several percent. With the new measurement device - EMF balance - a much better response characteristic is expected. EMF is a direct measurement technique whereas the pendulum setup is an indirect technique. 3 Ongoing work Two main steps are planned. The first is to investigate the pendulum setup in order to determine the major error sources to the total Lorentz force. For example the influence of heat fluctuations to the pendulum setup (see Fig. 2). For this, a heating coil will be applied to the channel in order to adjust certain temperature profiles during the fluid flow measurement. The second step includes further measurements with the new EFM prototype under variation of flow velocity, conductivity and flow profile of the fluid. Furthermore, it is planned to apply a sophisticated magnet system - so-called Halbach array. Numerical simulations suggested that the Lorentz force could be tripled with such an array system. That means, with this new array system it is possible to increase the distance between the two magnets from 32mm to 56mm and still have about the same Lorentz force.

5 4 Conclusion The applicability of Lorentz Force Velocimetry for the flow measurement of poorly conducting electrolytes is demonstrated using experimental results on salt water flow exposed to a permanent magnet system. The results provide a linear relationship between the measured Lorentz force, the flow velocity and the conductivity consistent with the theoretical scaling laws. Further measurements are currently being performed on a new experimental setup - an EMF prototype. The new experiments are aimed at investigating the effect of flow dynamics on the measured total Lorentz force. 5 Acknowledgment The authors are grateful to the German Science Foundation (Deutsche Forschungsgemeinschaft) for financial support of the presented work in the framework of the Research Training Group (Graduiertenkolleg) Lorentz Force Velocimetry and Lorentz Force Eddy Current Testing (GRK 1567/1) at Ilmenau University of Technology. References [1] A. Thess, E. Votyakov, B. Knaepen, O. Zikanov: Theory of the Lorentz force flowmeter. New Journal of Physics, 9, 299, [2] A. Thess, E. Votyakov, and Y. Kolesnikov: Lorentz force velocimetry. Physical Review Letters, 96 (164501), [3] Y. Kolesnikov, C. Karcher, and A. Thess: Lorentz force flowmeter for aluminum - Laboratory experiments and plant tests. Met. Trans.B., 42B ( ), [4] M. Werner and B. Halbedel: Optimization of NdFeB Magnet Arrays for Improvement of Lorentz Force Velocimetry to be published in IEEE Journal of Transaction on Magnetics, special Issue for INTERMAG Conference, [5] A. Wegfrass, C. Diethold, M. Werner, T. Fröhlich, B. Halbedel, F. Hilbrunner, C. Resagk, A. Thess: A Universal Noncontact Flowmeter for Liquids, Appl. Phys. Lett., 100 (194103), Biography: WEGFRASS, Andre; Dipl.-Ing.; Ilmenau University of Technology andre.wegfrass@tu-ilmenau.de